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[Pharmacology](https://durgical.com/en/pharmacology/) · Class

# Alpha blockers

Medicines that block alpha adrenergic receptors, relaxing vessels and the prostate. Used for benign prostatic hyperplasia, hypertension and pheochromocytoma.

Updated 2026-11-01

## What they are

Alpha-1 selective blockers (prazosin, doxazosin, terazosin, tamsulosin) relax arterioles and the smooth muscle of the prostate and bladder neck. The non-selective blockers phenoxybenzamine (irreversible) and phentolamine (reversible) block alpha-1 and alpha-2 and are used mainly for pheochromocytoma and, for phentolamine, extravasation of vasopressors.

## Mechanism of action

Blocking alpha-1 receptors prevents norepinephrine from contracting vascular and prostatic smooth muscle, so resistance and urinary outflow obstruction fall. Tamsulosin prefers the alpha-1A subtype of the prostate, so it lowers blood pressure less.

Target: Alpha-1 adrenergic receptors (alpha-1A in the prostate); alpha-2 for the non-selective agents

## Key points

- First-dose effect: orthostatic hypotension and syncope after the first dose or a dose increase.
- Intraoperative floppy iris syndrome during cataract surgery; the eye surgeon must know about alpha-1 blocker use.
- Additive hypotension with phosphodiesterase-5 inhibitors such as sildenafil.
- Priapism is a rare but urgent adverse effect.
- In pheochromocytoma, alpha blockade comes before any beta blocker, so unopposed alpha stimulation cannot cause a hypertensive crisis.

## Drugs in this class

- [**Prazosin** A short-acting alpha-1 blocker for high blood pressure. Its first dose can cause a sudden faint, so treatment always… Alpha-1 adrenergic receptors](https://durgical.com/en/pharmacology/prazosin/)
- [**Tamsulosin** An alpha-1A blocker that relaxes the prostate and bladder neck so urine flows more easily in benign prostatic… Alpha-1A adrenergic receptor](https://durgical.com/en/pharmacology/tamsulosin/)
- [**Doxazosin** A long-acting alpha-1 blocker for benign prostatic hyperplasia and high blood pressure, taken once a day. The first… Alpha-1 adrenergic receptors](https://durgical.com/en/pharmacology/doxazosin/)
- [**Phenoxybenzamine** An irreversible alpha blocker used before surgery for pheochromocytoma, to control the hypertension and sweating… Alpha-1 and alpha-2 adrenergic receptors (covalent bond)](https://durgical.com/en/pharmacology/phenoxybenzamine/)
- [**Phentolamine** A short-acting, reversible alpha blocker given by injection: the antidote when norepinephrine leaks from a vein, and a… Alpha-1 and alpha-2 adrenergic receptors](https://durgical.com/en/pharmacology/phentolamine/)
- [**Terazosin** An alpha-1 blocker for benign prostatic hyperplasia and high blood pressure. Like prazosin, it can cause fainting… Alpha-1 adrenergic receptors](https://durgical.com/en/pharmacology/terazosin/)

## Others in the class

- Alfuzosin
- Silodosin
- Urapidil

## Test yourself

Think of the answer, then tap the question to see it.

## What is the first-dose effect of alpha-1 blockers?

Marked orthostatic hypotension, sometimes with syncope, after the first dose or a dose increase.

## Why does tamsulosin lower blood pressure less than prazosin?

It is relatively selective for alpha-1A receptors, which predominate in prostatic smooth muscle, over the vascular subtypes.

## In pheochromocytoma, why is the alpha blocker started before the beta blocker?

Blocking beta-2 vasodilation first would leave the tumour's catecholamines acting unopposed on alpha receptors and could precipitate a hypertensive crisis.

## How does phenoxybenzamine differ from phentolamine?

Phenoxybenzamine binds alpha receptors covalently (irreversible, long effect); phentolamine is a reversible, short-acting competitive blocker.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [DailyMed - PRAZOSIN HYDROCHLORIDE capsule](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=7293cef3-ef9e-40dc-9995-875ef047f693) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - TAMSULOSIN HYDROCHLORIDE capsule](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=13a5032d-0be1-4001-916e-459f0b1a051d) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - PHENOXYBENZAMINE HYDROCHLORIDE capsule](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=0be6a394-cd10-42fc-97cf-ad53d81d8d93) (DailyMed (U.S. National Library of Medicine), 2026); Farmacología básica y clínica (Katzung), cap. 10 (McGraw Hill, 2025).

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